


On His Behalf

by sansual



Category: Undertale (Video Game)
Genre: College AU, F/M, Female Reader, Friends to Lovers, Gaster is an ASSHOLE in this okay, Light BDSM, Porn With Plot, Professor Kink, Sexual Tension, Swapfell Papyrus (Undertale), Teacher/Student, Threesome - F/M/M, au where the monsters never went underground, but like a hot one, love triangle? sorta
Language: English
Status: In-Progress
Published: 2018-10-24
Updated: 2018-10-24
Packaged: 2019-08-06 19:33:09
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,946
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/16393826
Author URL: https://archiveofourown.org/users/sansual/pseuds/sansual
Summary: Going to college is an experience in itself. Going to the same university as your best friend is a completely different adventure.You and your childhood friend, Rus, sign up for a magic-biology class together, taught by a Dr. W.D. Gaster. The professor's a skeleton, just like your best friend, but he's attractive in a whole different way than Rus is. Amidst feelings you didn't know you had, you start to want for your professor.It doesn't help that he seems to have it out for Rus. The lanky skeleton's having a hard time passing, and things aren't adding up.You can't let your best friend fail.What are you going to do to help him?(jk we all know what we're gonna do)





	On His Behalf

**Author's Note:**

> So this is gonna be a smutty two-parter that I got the idea for while spitballing with a very close friend of mine! She definitely helped me along, but I just loooove Swapfell Paps (as well as Gaster, to no one's surprise ever) and I hope I could do him justice.
> 
> Smut comes in during part 2. That being said, enjoy!

Going to college is an experience in itself. Going to the same university as your best friend is a completely different adventure.

You first met Rus in middle school, back when you were both so angsty and afraid of everything. Neither of you really fit in with the others, so you did what most of the loners did and grouped up together, just the two of you. The lanky skeleton and that one weird girl. 

You made it through high school like that, too. For prom you wore an indigo dress, and the tie he wore matched. It’s not like you stayed at the dance very long, though. The two of you were never the going-out type, much preferring to spend afternoons on the couch with a video game or two.

Other friends came and went, with their own agendas and drama, and it’s not to say that you didn’t enjoy their company. At the end of the day, though, it was always Rus driving you home.

You’d be lying if you said you never thought about dating him. He’s attractive in that slick, punk way that makes you stare from time to time. He’s sweet, too, almost surprisingly so, given his appearance and his rep. It’s not just to you, but to others, and his brother, too. 

The topic of dating has come up with you a few times over the years, always from someone else making a well-intended comment. 

“You two are so cute together,” they always say. “Have you even _thought_ about it?”

And Rus always knows how to respond. “we have, sure. but _____’s my best friend, and we don’t wanna risk losing that just to try something that may or may not work.” 

And he’s right, every time. 

Right.

On the day you are to sign up for freshman classes, the two of you pile together on your bed. It’s strange, almost, to think that you’ll be moved out of this room and into a dorm building in the next few months. One look at your skeleton friend, and you know he’s thinking the same sort of thing.

You and Rus both have to take a Magic-Biology class so you decide to get the same class time. It’ll be so much easier to tackle together. Thank the lord for core subjects. 

Unfortunately, though, your options are limited. Some of the classes are already full. Once you account for both that and the parts of your schedules you’ve already picked out, there’s only one slot that works for the both of you.

It’s a ten a.m., three days a week, taught by a Dr. W.D. Gaster. 

His rating online is... intimidating. Everyone says he's a hardass, that his tests are hell and he's kind of pompous. You're looking at the ominous comments with worry in your eyes. Should you and Rus just settle for different class times with nicer professors? After you do a bit of scrolling, Rus nudges you away from the screen.

“it’ll be alright, hon,” the lanky skeleton says. “even if it’s a shit class, we’ll suffer together.”

There’s something oddly sweet about that statement. It reassures you enough to stick with the class. 

 

The summer passes without incident. If anything, it only makes you more and more eager for the first part of your adult life. Rus moves in a day after you do, but he’s still there to help you carry and rearrange things. 

The first day of college is a Monday, and you and Rus walk to your ten a.m. together. Dr. Gaster's class is in a rather large lecture hall, already abuzz with small talk. The two of you stride through the room five minutes early and sit right next to each other, near the middle. 

The room is cold. Your leg bounces incessantly. 

It’s almost as if Rus can sense your anxiety. “seriously, relax. it won’t be bad. we’ve got this.” His bony thumb ghosts over your arm, barely there but still comforting. You always appreciate his light touches. 

With a loud _tick_ , the stark-white analog clock stands at ten o'clock exactly. An anxious silence settles over the class.

Then a door at the front of the room opens, and out steps Dr. Gaster.

Your professor is, like Rus, a skeleton, standing taller than any human man. His skull is squarely-angled, almost like that of a human face, with two scars branching out from his sockets like cracks on a windshield. Even though monsters don’t typically show or measure age very obviously, he seems older. Perhaps if he were human, he’d be in his late forties. 

Nevertheless, he's _very_ attractive, though in a different manner than the way you find Rus attractive. He’s wearing a fitted turtleneck under his suit jacket, and everything is in greyscale. It’s a polished, classy look that you appreciate.

He steps to his podium in the lecture hall, footsteps tapping timely against the floor. A long, gloved hand reaches down to the mic to adjust it, giving it a few taps to ensure it’s on. 

And then he says, “Good morning,” and that voice is so deep and dark you could drown in it. 

_Oh dear god._

Rus taps your shoulder, leans in, and asks, “you okay? lookin’ a bit scared over there, darlin’.” 

You’re not sure how to answer that. 

“Excuse me, young man,” Dr. Gaster’s voice interrupts your thoughts. When you look up at him, he’s staring Rus down. That glare is icy. “Are you speaking over me? In my own class, on the first day?” 

“shit, sorry,” Rus quickly defends himself.

Your professor only clicks his long, inky-colored tongue (that you _definitely_ aren’t ogling). 

“You’re paying for this class, so I suggest that you listen. Besides, I doubt that the lovely lady beside you would appreciate you distracting her.” 

He looks at you and smiles. 

In the seat next to you, Rus feels white-hot jealousy bubbling up inside of him. 

 

It’s all downhill from there, frankly. The class isn’t that hard for you, much to your surprise. You speak actively and take notes diligently. You’re always there, too. Every time Rus suggests skipping, you merely grab him by the hand and drag him to class. 

Dr. Gaster seems to appreciate your efforts immensely. He calls on you frequently, and when you get especially emphatic about an answer you see a trace of a very dark smile on his skull. A look like that never fails to make you flush in that lecture hall. He’s handsome, too handsome. Some days you can’t think straight, and yet, you’re doing impeccably.

You get your first exam back and you’re thrilled with the high B you made. It’s especially pride-inducing when you realize that most of the class failed.

Rus included. 

You’re baffled when he lets you see his grade. 

“How in the world…” you ask. The two of you did all of your studying together, and he’s been to every class, just like you have. How could he put in the same amount of effort and get such awful results? Rus isn’t stupid, not by a long shot.

The lanky skeleton’s convinced that Gaster has it out for him, and you’re not disagreeing. When you go over the exams in class the next day, Rus lets you see his copy. His questions are… harder? They cover roughly the same material, but the phrasing is so much more complex, and so much more difficult to understand. Sentences swirl and loop around each other. It takes you several minutes to understand what they’re even asking. There are some questions your friend didn’t even answer, they were so difficult.

“Go talk to him after class,” you whisper from your desk. “This is _ridiculous_.” 

“nah,” he replies, raspy voice low. “i got somethin’ better."

You can only fix him with wide, bewildered eyes as he raises his hand. 

Seeing the motion, Dr. Gaster turns in your direction. “Yes, Rus? May I help you?”

“uh, yeah, what the fuck is this?” He’s got his test gripped tight in his phalanges, holding it up next to him while he stands. 

“Rus,” you stage-whisper, giving him as serious of a side-eye as you can manage from your seat next to him, “What the fuck are you doing? Sit _down_!”

He ignores you. Gaster glares him down. 

“What do you mean?” Your professor contests. “If you’d like to discuss your grade, Rus, you may do so after class.” 

Your friend only snorts through his nasal ridge. “nah, i’d like to discuss you. i'm callin’ bullshit.” 

You’re not oblivious to the way Dr. Gaster grits his teeth, malleable mouth curling into an irate sneer. 

“On what, exactly?” 

_“you.”_

“Pardon?” Though he’s asking for clarification, you can tell he doesn’t need it. It’s an out. _Back down, Rus. Please._

Of course, he doesn’t. He spits it. “i said, you’re fuckin’ bullshit. it’s all you are."

You can hardly get out an “Oh my god, Rus,” before Gaster begins to step closer, closer to where you’re sitting together, as close as he can get. There’s no sound in the air but the precise, evenly-timed clicking of his shoes against the tile. Only a few tables serve to separate him and Rus, and you’re so thankful for them. You’re not even the subject of his anger and yet you’re still trying to shrink into your chair. The sneer’s worsened. You can see every meticulous point of his perfect teeth. Dr. Gaster’s eyes burn into Rus, molten hot blackness, dark and getting darker still. 

The worst part is how steady he still sounds. At the sharp edges of his voice is a cold hiss. “You _dare_ to disrespect _me_ in _my own class_? To _embarrass_ me like a damned _fool_? It seems you’re more of an _idiot_ than I perceived.”

Rus opens his mouth, but he’s cut off.

“Be surprised if you pass, you incorrigible nuisance of a boy,” Dr. Gaster snarls, and then turns on his heel and walks away. 

All you can do is put a comforting arm around Rus and try to ignore the burning between your legs. 

 

The instant you get out of class, you whirl on your best friend. “Rus, what the hell?!” Your hands find his shoulders and squeeze hard. His jacket’s soft.

“had to call him out somehow. that’s all i’m gonna do. worst-case scenario, now everyone knows he’s a fuckin’ ass.”

“An ass?” You ask. “He looked like he was going to fucking kill you! Holy shit, and that thing he said about failing?”

“empty threats, hon, empty threats,” he waves a bony hand off to the side as you walk together. 

You sigh. “Rus, he scared me.” You’re not going to mention what _else_ Dr. Gaster makes you feel, especially not after what just happened. Not a good time.

But Rus coughs out a dry laugh. “you’re the last person who should be scared of him. you’ve got the perfect grades, you’re always talking, and oh, he definitely wants to _bone_ you.”

“What?” You snap your head in his direction. What?

“uh, is this one of those times where you want validation, or do you really not notice how he looks at you _every damn day_?” 

No, you've seen the looks, those sly smirks and intense gazes, but they’re always directed at the class in general, right? And even if they _were_ directed at you, they don’t mean… do they? 

Rus seems to notice your internal crisis, because he stops and pulls you to the side. There’s a bench by the sidewalk, and with both his hands he plops you down on it. The wood creaks when he sits down next to you.

“darlin. relax,” Rus tells you. “don’t worry about it. don’t worry about _him_. i can handle this.”

But can he? He’s been struggling with every assignment so far, and Dr. Gaster _certainly_ isn’t making things any easier on him. What happened in class was _intense_. Almost one half of the semester is already over, and aside from the stunt he just pulled, you have a hunch that Rus isn’t going to do anything else about his grades or the class. That’s how he typically “handles things,” anyway. 

_”Be surprised if you pass,”_ Dr. Gaster told Rus. Given how straight-forward he’s been, you don’t doubt that he meant it. The thought’s terrifying. An F on Rus’ transcript already? He’s got to keep his scholarship. He can’t flunk out, he’s too smart, too kind. This isn’t fair.

You aren’t going to let your best friend fail while you have an A. Surely there’s something you can do. 

When you get back to your room later, you look up Gaster’s office hours, and learn that he does appointments by phone. Oof, you’ve always hated making calls. Nevertheless, though, you work up the courage, suck in a breath, and dial the number your professor listed.

Ring. Ring. What will you do if it goes to voicemail? Ring.

“Yes?” The voice that answers is smooth, too smooth. You’re feeling warm again. 

You try not to let your voice squeak. “Dr. Gaster? It’s _____, from your 10 a.m. magic-biology?”

“Ah, yes, my star student.” Something about that phrasing sends shudders through your whole body. “What do you need, dear?”

_What do you need?_ There are a lot of answers to that question, especially when coming from him. You need to figure out what’s happening with Rus’ grade. You need to see what you can do to help him. You need to know if your professor wants to...

You take a deep breath, and then say into the phone, “Can we meet?”

 

You don’t tell Rus, not a damn thing. Two days later, you get up and fish that little black skater dress from the back of your closet. You pair it with cute knee-highs. There, it’s dressy, but not over-the-top. A spritz of your favorite perfume wouldn’t hurt, either. 

Office hours are right after class, and you’ve got the first slot. You can do this. You’ll do it for Rus. You’re antsy the whole time, though, and it doesn’t help that your best friend is suspicious. 

“what’s with the dress, hon?” He asks as you meet to walk to class. You watch those low, lidded eyes travel up and down your body. His gaze is a welcome one, with familiar warmth behind it. “not that ‘m complainin, makes you look super cute. but still, you got a date or somethin’?” 

“Pfffft, no,” you snort, laughing it off. “Just felt like it today.” 

He doesn’t pry any more, but you don’t miss the way he keeps stealing glances at you. 

If that weren’t enough, there’s also the fact that Dr. Gaster is _most definitely_ staring at you during class.

He’s subtle about it, sure, but you can’t overlook the way that intense violet gaze keeps flicking over to your seat. It makes you heat up all over in a way that’s not exactly unwelcome. It’s a struggle to not squirm in your seat.

_Don’t freak out now. This is normal. Rus said he normally stared at you all the time in class. Maybe this is just the first time you’re noticing it. It probably doesn’t even mean anything. He’s waiting for you to raise your hand to talk again, or he wants to know what you think about the lecture. Maybe today he’s wondering about the meeting. The meeting isn’t even a big deal, either. You’re just going to talk to him and kindly convince him to lighten up on Rus somehow. If you’re polite, and well-mannered, and persuasive enough, you can probably make some progress._

Rus pulled quite the stunt last class, though. How in the world are you going to persuade Dr. Gaster to forgive _that_?

Class gets out, and you run into your next problem when Rus starts talking about grabbing lunch. 

“…there’s the cafeteria, that’s an idea. or there’s the burger joint across the way, i know you love their cheese sticks, or-"

“Actually,” you butt in, “I, um, I have to run an errand and it’s not something I can really put off. If you still want we can get food after I’m done!” 

Raised sockets and a contemplative frown. Yeah, no, he doesn’t buy that at all. He knows you too well, and at the very least enough to grasp when you’re lying to him. 

But all he says is, “alright. whatever you say. i’ll, uh, i guess i’ll text you then. see you around, hon.” The smile he gives you is too forced. 

You feel bad, but you know that if he knew he wouldn’t let you go. He already can’t stand Gaster to begin with, that much is obvious. And then there’s how you're dressed, and then there’s the possibility that your professor wants _something else_ from you… Your best friend would have a stroke if he knew where you were going and what you were doing.

So you watch him go, and when he’s several steps away you turn and make your way towards the concrete building across the block. Your meeting’s in just a few minutes. At this pace, you’ll be right on time.

Rus at least lets your figure get a good way down the street before following you.

 

Dr. Gaster has an office all to himself. You stand outside the door for a few moments, just taking some breaths. You’ve never spoken to him one-on-one before, or at least not without an audience. It’ll be fine, you tell yourself. You’re his… star student. That’s what he called you, though the underlying tone of that was and is questionable. He likes you, or at least appreciates you. You’ll be fine. The door looks unlocked, but you knock anyway.

“Come in.” That voice seems even deeper when it’s not echoing off the walls of a lecture hall. It pulls you through the door and into your professor’s office. 

Gaster’s sitting behind his desk in a very comfortable-looking chair. There’s an ice-filled glass of some amber liquid on a coaster in front of him, and you have a feeling it’s neither Coke nor Pepsi. How did he manage to get so settled in his office when class just ended?

You’re a little intimidated. Okay, extremely intimidated. 

“Good afternoon, _____. Please, take a seat, make yourself comfortable.” 

Your throat’s going dry and your body’s heating up. You sit. 

Gaster says, “By the way, you do look quite nice today. It’s a lovely change from those tight leggings you seem to enjoy.” 

Are they tight enough for him to notice? Does he notice? Does he stare when you aren’t looking? Rus seems to think he stares. Your face is getting warmer.

“Thank you, sir,” you stammer. He seems to like that, and smiles. Oh, that expression. “But anyway,” you continue, “I’m here to discuss something with you.” 

He clicks his tongue, gaze level with yours. “This is about Rus, is it not?” It’s not a question. He already knows why you’ve come to him. 

You nod. “Look, Dr. Gaster, he’s my best friend. We do everything together. We signed up for this class together, take notes together, and do all of our studying together. It doesn’t explain why I have an A and he’s nearly failing. It doesn’t add up.” 

“A correct assumption,” he remarks. 

“And then there was what happened the other day in class. Sure, Rus started the ruckus, but you seemed to spur him on and encourage his behavior. You built it up until it was reasonable for you to lash out at him and threaten his grade,” you reason aloud. It’s taken a while for you to process it all, but now that you have, you feel like it’s coherent.

“What are you getting at?” Gaster asks. He’s got one elbow propped on the mahogany, peering at you inquisitively. There’s a darkness to his voice that only intensifies the heat in the room. 

Is it the room, or is it you? 

But then you lean forward on the desk. You’re probably going to regret being this forward. “What I’m getting at is that I notice how you treat him. I think you’re giving him a hard time on purpose. You want an excuse to fail him.” 

And it’s out there.

His smirk is almost predatory. It’s terrifying (exhilarating). “Such a bold conclusion, my dear.”

A beat of silence, a beat of tension. Your skirt’s too tight. You narrow your eyes and lean closer.

Gaster says, “And it is a correct one. But why wouldn’t I challenge Rus? He’s blatantly rude, disrespects me at every turn, and then he distracts *you* from *your* education. Come now, don’t you see that he hinders you?”

“I don’t see that at all.”

He seems almost amused by your curt reply. “Nevertheless, his massively-inflated ego could afford to be knocked down several notches.” 

“You’ve knocked him down enough, I’d say,” you remark.

“My, my, are we talking back?” He sips his drink (you think it’s scotch or whiskey of some sort) but he keeps his eyes trained on you the whole time. You watch him swallow the liquor without the slightest change to his expression.

“For my friend, I would.” 

He stops a moment to laugh a little, haughty silk coming out his jaws. “That’s the second time you’ve used the word ‘friend’ to describe him. My dear, you’re so intelligent; do you really not see how much he adores you?” 

You fix him with furrowed eyebrows and a flatlined mouth. He continues.

“Frankly, I’m surprised you aren’t out wearing that little dress for him instead. A beautiful woman like yourself deserves that kind of attention he wants to give. That boy looks like a starved animal in class, he’s so desperate for you. It’s almost tragic how funny it is.” 

Rus… likes you? Enough for Gaster to notice? And you’re… beautiful?

But Rus said that _Gaster_ was the one that liked you. 

“What-“

“But never mind that,” Dr. Gaster brushes off the topic with a wave of his hand. “I’d like to know why _you_ , lovely girl, are here on his behalf.”

Another beat. You’re shaking, struggling to keep your cool in a room that’s so hot, with a man that’s making you hot on the inside. If he calls you lovely or dear one more time, you just might keel over. 

He prompts, “Or is there perhaps another reason you’re here instead?” 

The implications are clear. They’re crystalline, with that gaze of his. You haven’t been blind to the way it’s been traveling up and down your body slowly, like you’re a meal he’s savoring. He could eat you alive, devour you and leave a wanton mess in the wake of your body.

You’re trying to get your composure, but your voice keeps shaking. “I’m- I’m here for Rus, to help him, but also I…"

Shaking fingers curl around the hem of your little dress. Your breath catches in your throat. Are you really about to…

Are you going to act like you don’t want to?

You hitch your skirt up, just enough, and lift one of your knees until it’s up and flush on his desk.

“I think I know how to appease you, sir.” 

He looks pleased already, that wicked grin widening across Gaster’s scarred skull. After taking another sip of his drink, he pats the condensation off of his gloved hands onto his slacks.

“Ah, I was wondering when you would breach this. Brave girl.” 

And then his voice drops to something even deeper, and that refined cadence now has a razor-sharp edge to it.

“You can start by crawling to me.”

**Author's Note:**

> Oh shit we gon' crawl.  
> Part 2's already in progress and should be out soon. I hope you're ready because I'm sure not.
> 
>  
> 
> [My Tumblr](https://www.tumblr.com/blog/sansualfics)
> 
> [Tip Jar](https://ko-fi.com/sansual)


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